English: Fossil snail from the Pennsylvanian of Illinois, USA.

Of all the molluscs, the gastropods (snails) have made the most ecological adaptations. They can be found in almost all fundamental environments: marine, freshwater, terrestrial. Most gastropods live in the ocean, and have a single, asymmetrically coiled, external shell of calcium carbonate (CaCO3 - usually aragonite). The hard calcareous shell is the most easily fossilized part of the gastropod. The soft parts of a snail (the “slug” portion) include a well developed head having eyes, tentacles, and a mouth, and a well developed, strong, muscular foot used principally for locomotion. The shell is carried upright on the snail’s back, or is partially dragged behind. When threatened by a predator, many snails can retract their soft parts into the shell’s interior for protection.

Many fossil snails in the Paleozoic rock record are often not well preserved, or are preserved as internal molds. The original aragonite of many gastropod shells is not stable on geologic time scales, and often recrystallizes or dissolves completely away. Fossil snail shells in Mesozoic and Cenozoic rocks are usually better preserved.

Classification: Animalia, Mollusca, Gastropoda

Stratigraphy: Modesto Formation, lower Missourian Series, lower Upper Pennsylvanian

Locality: stream cut along an east-flowing tributary of the North Fork of the Embarras River, south-southeast of the town of Casey, southwestern Clark County, eastern Illinois, USA (= stop 5 of Weibel, 1991)

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Reference cited:

Weibel (1991) - Sequence stratigraphy in mixed clastic-carbonate strata, Upper Pennsylvanian, east-central Illinois. Great Lakes SEPM 21st Annual Field Conference Guidebook.